1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor and an image forming apparatus including the same. More particularly, the present invention relates to an electrophotographic photoreceptor in which a photosensitive layer contains oxygen-containing fluorinated fine particles in a surface layer thereof, as well as to an electrophotographic photoreceptor in which a photosensitive layer contains oxygen-containing fluorinated fine particles in a surface layer thereof and the photosensitive layer contains a charge transport material having an ionization potential of 5.25 to 5.70 eV, and to an image forming apparatus including the photoreceptor.
2. Description of the Related Art
In electrophotographic image forming apparatuses that are used as copying machines, printers, facsimile machines, and the like (hereinafter, referred to as electrophotographic apparatuses), an image is formed through the following electrophotographic process.
First, a photosensitive layer of an electrophotographic photoreceptor (hereinafter, may be referred to simply as “photoreceptor”) included in an apparatus is uniformly charged at a predetermined potential by a charger.
Subsequently, the photosensitive layer is exposed to light such as laser light emitted by exposure means according to image information, thereby forming an electrostatic latent image.
A developer is supplied from developing means to the electrostatic latent image formed, and a component of the developer, that is, colored fine particles referred to as toner is attached to the surface of the photoreceptor. Thus, the electrostatic latent image is developed into a visible toner image.
The toner image formed is transferred from the surface of the photoreceptor to a transfer material such as recording paper by transfer means and fixed thereon by fixing means.
However, not all the toner on the surface of the photoreceptor is transferred to the recording paper in the transfer by the transfer means, but some of the toner is left on the surface of the photoreceptor. In addition, some paper particles from the recording paper having been in contact with the photoreceptor in the transfer may adhere to the surface of the photoreceptor and remain thereon. Having an adverse effect on the quality of an image to be formed, such foreign matters as the residual toner and the adhering paper particles on the surface of the photoreceptor are removed by a cleaner.
In recent years, furthermore, there have been technological advances toward a cleaner-less system, and the foreign matters may be removed with a developing and cleaning system, in which the residual toner is recovered by a cleaning function added to the developing means without using independent cleaning means.
According to this method, charges on the surface of the photosensitive layer are removed by a discharging device after the surface of the photoreceptor is cleaned, and then the electrostatic latent image is eliminated.
An electrophotographic photoreceptor that is used in such an electrophotographic process has a configuration including a photosensitive layer containing a photoconductive material stacked on a conductive substrate made of a conductive material.
As the electrophotographic photoreceptor, photoreceptors formed from an inorganic photoconductive material or an organic photoconductive material (hereinafter, referred to as organic photoconductor, abbreviated as OPC) may be mentioned. Since recent research and development has improved the sensitivity and the durability of organic photoreceptors, the organic photoreceptors are more commonly used today.
In terms of the configuration of the electrophotographic photoreceptors, multilayered photoreceptors have been recently mainstream photoreceptors, in which a photosensitive layer includes functionally-separated layers: a charge generation layer containing a charge generation material and a charge transport layer containing a charge transport material. Most of the photoreceptors are negatively chargeable photoreceptors in which a charge transport layer formed from a charge transport material having a charge transport ability molecularly dispersed in a binder resin is stacked on a charge generation layer formed from a charge generation material vapor-deposited or dispersed in a binder resin. In addition, monolayer photoreceptors have been proposed, in which a charge generation material and a charge transport material are uniformly dispersed or dissolved in the same binder resin.
Furthermore, in order to improve the quality of an image to be printed, an undercoat layer may be provided between the conductive substrate and the photosensitive layer.
Disadvantages of an organic photoreceptor include surface wear caused by slide and brush of a cleaner or the like on the periphery of the photoreceptor due to the nature of organic materials. In order to overcome the disadvantage, an attempt has been made so far to improve mechanical properties of the material of the surface of the photoreceptor.
It has been an important challenge to achieve extended life and higher image quality as desired functions of photoreceptors.
In order to achieve extended life of photoreceptors, it is necessary to improve the wear resistance and ensure the potential stability and the high image quality.
As solutions for achieving extended life, there have been known a method by providing a protective layer to an outermost surface layer of a photoreceptor to give lubricity (for example, Japanese Unexamined Patent Publication No. HEI 1(1989)-23259) and a method by including filler particles in a protective layer (for example, Japanese Unexamined Patent Publication No. HEI 1(1989)-172970). In such methods, it has been considered to add fluorinated particles to the surface as a filler (for example, Japanese Patent No. 3148571 and Japanese Patent No. 3416310). Having a high lubricating function derived from their material, as one of their characteristics, the fluorinated particles as a filler not only improve mechanical properties of the photoreceptor but also reduce the friction between the photoreceptor and a member in contact with the photoreceptor during the process by giving the photoreceptor lubricity. Thus, the fluorine particles contribute to improvement of the printing durability of the surface of the photoreceptor.
Fluorinated fine particles such as, for example, polytetrafluoroethylene (PTFE) particles have an excellent lubricating function as a material. However, the PTFE molecule forming the fine particles does not have polarity, and therefore the fine particles have a very large particle-to-particle attraction force. The fluorinated fine particles are therefore disadvantageous in that they show extremely poor dispersibility in the preparation of a dispersion of the fine particles. Accordingly, it is necessary to use a dispersant when PTFE fine particles are dispersed to be used for a photoreceptor (for example, Japanese Patent No. 3186010). As a result, use of the PTFE fine particles deteriorates electric properties of the photoreceptor.
In addition, as the photoreceptor lives its extended life, the surface (in particular, the charge transport material) of the photoreceptor deteriorates due to the pollution from NOx or ozone gas generated when the photoreceptor is charged, causing a defect in the quality of an image being obtained such as image blurring.
In order to achieve higher image quality in the photoreceptor, a charge transport material having high oxidation resistance and generally having high ionization potential can be selected. In this case, however, charge injection to the charge generation layer and the charge transport layer is difficult, and the sensitivity tends to decrease. Accordingly, use of such a material in combination with the above-described fluorinated fine particles, which are effective for the printing durability improvement, causes further sensitivity reduction in addition to the deterioration of the electric properties of the photoreceptor due to the use of a dispersant.
On the other hand, use of a charge transport material having better responsiveness and electric properties but generally having low ionization potential in combination with the above-described fluorinated fine particles, which are effective for the printing durability improvement, can inhibit the deterioration of the electric properties of the photoreceptor due to the use of a dispersant but causes a defect in the image quality due to the pollution from NOx and ozone gas, because the material has poor oxidation resistance as a charge transport material.
At present, as described above, there has not been found a satisfactory solution to the challenge to achieve both extended life and higher image quality.